The detection of data in a communications system, radar system, or other type of data-receiving system can be achieved with various demodulation systems. A demodulation system typically consists of a bandpass filter (BPF) which separates the received signal from the noise components outside of the desired frequency spectrum, a detector to recover the baseband signal together with other signal harmonics, and a lowpass filter (LPF) which separates the baseband signal from the higher frequency components of the detected wave.
There are two common modes of detecting data in an AM-modulated signal. These are coherent and non-coherent detection. Coherent detection is also called homodyne, or synchronous detection, while non-coherent detection is called non-synchronous, or envelope detection. Coherent detection requires that the characteristics of the received signal be known, but this requirement is not necessary for non-coherent detection. Coherent down-conversion in radar applications leaves an error in the detected radar return which is proportional to the velocity and the orientation of targets. Non-coherent down-conversion, or envelope detection, leaves errors in the determination of target size and location. Calibration and drift of the envelope detection circuitry at high radar bandwidths is also a significant problem.
Coherent detection is based upon carrier synchronization of the received signal with the signal from a local oscillator. The received signal, after passing through a bandpass filter, is multiplied with a signal of the same frequency and phase from a local oscillator to produce a multiplied signal on the output of the miltiplexer.
If the incoming signal is represented by: EQU S*(t)=A(t)cos .omega..sub.c t,
where S* is the instantaneous value of the carrier signal at time t and the local oscillator is represented by: EQU S(t)=cos .omega..sub.c t,
The multiplication process thus produces the signal: ##EQU1## where .omega..sub.c is the angular carrier frequency and A(t) is the data baseband modulating signal. This mixed, or multiplied, signal is then fed to a lowpass filter which removes the second harmonic of the mixer output signal and leaves the desired baseband signal component: EQU 1/2A(t)
While coherent detection systems have a lower error rate probability than non-coherent systems, the fact that the local oscillator signal must be matched to the frequency and phase of the received signal makes the system impractical for certain applications, such as radar return signals which have unknown modulation, carrier and phase components.
Non-coherent detection systems can achieve the same probability of errors as coherent detection systems, but only at the expense of higher signal-to-noise ratio power. Non-coherent detection is based on envelope detection of the received signals. As in the coherent detection the received signals may first be sent through a BPF. The signal then passes through a limiter to insure that the received signal is of a constant amplitude, and it is then sent to a differentiator circuit. Following differentiation of the signal, an envelope detecting circuit receives the signal. The output of the envelope detecting circuit is supplied to an analog-to-digital converter which converts the signal to digital data.
Extraction of the carrier reference frequency from the received signal to provide a local oscillator signal for coherent detection has been accomplished by passing the received signal through the BPF to a frequency doubler which typically is a square-law modulator. If the received signal is: EQU S*(t)=A(t)cos .omega..sub.c t
the output signal from the frequency doubler will be ##EQU2## The signal is now passed through a highpass filter (HPF) so that only the second harmonic of the carrier signal is retained. The remaining signal is then sent through a limiter and a frequency divider to produce the required carrier output signal EQU .+-.cos .omega..sub.c t
Production of a carrier reference recovery signal in this manner is not adequate when phase modulation occurs because frequency doubling of the transmitted signal will double both the carrier frequency and the phase changes. In order to recover the carrier signal when phase modulation occurs, phase lock loops of various types are generally employed. In radar systems, the AM return signal contains a carrier reference only during target reflections. Therefore, phase delays in the frequency doubler, limiter and frequency divider make this approach unacceptable in carrier recovery in such systems.
The above discussion of data transmission demodulation of coherent and non-coherent systems and carrier frequency recovery is based upon information found on pages 120-125 of the book Communication for Command and Control Systems by D. J. Morris, Pergamon Press, Maxwell House, Fairview Park, Elmsford, N.Y. 10523, .COPYRGT.1983.
A product brochure from Harris Semiconductor, dated March 1995, for their Gilbert cell UHF transistor array product designated as HFA 3101 refers to the possible use of this product as a frequency doubler. This product has two terminals that can serve as modulating and carrier ports of a modulator. The brochure indicates that if the same input signal is fed to both ports, the output frequency will be the sum of the carrier frequency (.omega..sub.c) and the modulating frequency (.omega..sub.c), and that this is equivalent to twice the input frequency. In addition, it notes that a D.C. component equivalent to the difference of .omega..sub.c and .omega..sub.m is provided. The Harris brochure also shows that zero IF down conversion data demodulation may be obtained with the HFA 3101 circuit by coupling a local oscillator to the carrier, or RF, port and a modulated input signal with the same carrier frequency to the modulating, or L0, port.
An article entitled "Principles of Digital Communication" published in Electro-Technology, February 1967, pages 75-84, by P. A. Wintz and R. E. Totty, describes on pages 77 and 78 a system which may be utilized for determining the average power of a nondeterministic waveform. In this system the waveform is first passed through a bandpass filter to a squaring circuit. The squared output waveform from the square is then passed through a lowpass filter to a DC meter. The lowpass filter is utilized to supply a DC signal average total power. The filter minimizes fluctuations that occur at the DC meter. The intent of this circuit is to substantially capture the average DC component and there is no attempt in the system to extract any information or to provide any demodulation of the waveform.